The Hypoxia Inducible transcription Factors (HIFs) are induced in a large number of cancers where they upregulate the expression of over 100 genes, many of which are expected to promote tumor progression and confer resistance to traditional therapies. The goals of Project 4 are the elucidation and validation of targets for the discovery of small molecules that modulate this response with the objective of using these reagents to assess the contribution of HIF to tumorigenesis in in vivo model systems. Biologically sound cell-based high throughput screens exploiting knowledge of oxygen-dependent modes of HIF regulation have led to the identification of both siRNAs and potent (nanomolar) small molecules that either antagonize or agonize the pathway. Though the molecular targets for some of these agents have been successfully identified, Aim 1 proposes biochemical, molecular biological and genetic approaches to pursue the modes-of-action of the remaining compounds as their targets may represent unidentified components of the hypoxic response pathway. Concurrently, structural, biochemical, and biological approaches have rigorously characterized the HIF Per-ARNT-Sim (PAS) domains responsible for mediating interactions between subunits of the HIF heterodimer. Aim 2 extends these studies to investigate a working model for HIF assembly. Using such information, proof-of-principle ligands for these PAS domains have been identified and shown to function as allosteric inhibitors of HIF heterodimerization in vitro and provide justification for the development of new, high throughput, assays for future screens. Furthermore, the best compounds from each Aim will be subjected to systematic chemical efforts to improve their potency, stability, and solubility for use in studies to interrogate their efficacy in in vivo models of human diseases. These complementary routes offer unique opportunities to gain insight into HIF function and mechanisms by which it may be artificially regulated. Relevance to public health: Increased levels of HIF are observed in several human tumors and correlated with tumor aggressiveness, resistance to treatment and mortality. In addition, HIF is induced in several diseases in which oxygen availability is compromised, including anemias and ischemias. As such, small molecule modulation of HIF activity may have therapeutic utility in a range of diseases including cancer.